Rectangular mortise and tenon construction has been widely used in
barns and other buildings in this and other countries, and there is
along history of its performance. Round mortise and tenon joints are
essentially a modem variation of this construction. They are much easier
to manufacture because 1) the tenons can be cut with hole saws; and 2)
the mortises can be drilled rather than chiseled. As a result, this
method of construction lends itself to mass production techniques. Using
this construction, a variety of modular building frames can be
constructed "tinker toy" style from are relatively small set
of standardized parts. Potentially, this system provides an outlet for
small-diameter tree stems that are presently of essentially no economic
value, but more importantly, it provides the basis for solving numerous
housing and other building shortages in underdeveloped regions at the
cottage industry level. To obtain insights into the specific
characteristics and peculiarities of round mortise and tenon join t
building frame construction, a small building was designed and
constructed utilizing this system. A description of the building and the
lessons learned from its construction are given in this paper.

**********

According to Wolfe (2000), many forest stands in this country are
overstocked with small-diameter trees. This material poses both a health
hazard and a fire hazard, and the excess stems should be selectively
thinned, but they presently have too little market value to justify
their removal. A similar situation exists in many developing countries
that have replanted denuded forest areas (Ramirez 1998). Thus, large
quantities of potentially useful but largely unused wood materials
presently exist and are available for conversion into value-added wood
products, but these products must be of sufficient value to justify
harvesting costs. As pointed out by Wolfe (2000) and Wolfe and Moseley
(2000), structural applications offer a high-potential value-added
market for small-diameter stems. From a structural perspective, however,
several problems must be solved. In particular, strong yet
cost-effective methods must be found to join the members together
(Eckelman 1995, Karlsen 1967, Rug and Potke 1990).

Studies in related areas (Eckelman et al. 2000) suggest that round
mortise and tenon joint construction may provide a partial answer to the
problem of the structural joining of small-diameter tree stems
economically. Specifically, round mortise and tenon joints are easily
constructed and lend themselves to the fabrication of standardized parts
that can be incorporated easily into modular constructions. Furthermore,
the tenons themselves are efficient load carriers (Eckelman 1970) and
are highly resistant to cyclic loading in beading (Haviarova et al.
2001a, 2001b).

It is recognized, however, that the bending strength of round
mortise and tenon joints is limited by the strengths of the tenons
themselves; if higher strengths are needed, there is no simple way to
reinforce the joints. Thus, building frames must be designed in such a
way that internal bending forces are distributed as uniformly as
possible to the various members and joints. In addition, a sufficient
number of members must be included in the design to ensure that the
total internal strength of the construction is sufficient to carry the
loads imposed on the structure. The inherent nature of these designs is
such, therefore, that numerous small members should be used to form a
construction rather than a few large members.

It must also be recognized that the use of tight fitting joints
would make joining of members difficult. In practice, therefore, round
mortise and tenon joints cannot be expected to have high tensile
strength, and, in general, should not be designed to function as primary
tensile load connectors in building frame constructions. In general,
therefore, frame designs must be developed that avoid tension loading of
tenons. Pilot studies, however, indicate that relatively high levels of
withdrawal strength can be obtained with pinned tenon joints, in those
cases where tension loading of joints cannot be avoided. In these
joints, a cross pin of half the diameter of the round tenon is inserted
crossways in a hole drilled through the tenon and the member it joins
together. It is anticipated that these joints would provide sufficient
strength for secondary joints such as those used to anchor roofs against
the uplift forces created by wind loads.

The variability anticipated in the strength and condition of the
cores of small stems also dictates that some type of pre-treatment would
be needed before tenons could be cut on the ends of some members. The
inside/outside cut and glue process of Serrano (1999) in producing
nominal 4 by 4s from 2 by 4s cut from small stems (in which the core
material is placed on the outside faces of the 4 by 4s) provides one
means of obtaining sound cores from small stems. In the case of smaller
stems, tenons can be cut that are of about the same diameter as the
small end of the member, thus preserving the strength of the material
outside of the core area.

It must also be recognized that for many applications, round
timbers would need to be squared before they could be used. Although
this can be done with a small portable band mill, a Scragg saw would be
needed for a large-scale operation.

Preservative treatment of the wood would be important in areas
subject to termite infestation or where the wood would be used in
applications where it is frequently wetted. Ideally, the wood should be
treated after tenons and mortises have been cut.

Finally, building construction is a very conservative trade and
change is slow because the adoption of new methods represents an
investment of both time and capital. New materials and systems,
therefore, not only need to meet basic strength and durability
requirements, but the systems need to be as simple as possible to reduce
the need for specialized labor (Ramirez 1998, Leandro 1996). Acceptance
of round mortise and tenon construction on a regional basis requires the
standardization of parts and components to ensure that they will fit
together when brought to a building site. Widespread use of the system
also dictates that simplification and standardization of components must
occur without unduly limiting design possibilities or dictating the
nature of the building that can be constructed from them (Jensen et al.
1990). Thus, although round mortise and tenon joints can be used in
one-of-a-kind unique constructions, there is strong need for the
rational development of standardized components that can be used to
fabricate a wide variety of frame systems.

Rectangular mortise and tenon construction has been widely used in
barns and other buildings in this and other countries, and there is a
long history of its performance (Sloane 1967). Round mortise and tenon
construction differs essentially only in the geometry of the joint, but
information is lacking concerning its use in building frame
construction, although the use of dowels as connectors has been
investigated (Stern 2001). Information is needed, therefore, concerning
the basic characteristics and the idiosyncrasies of round mortise and
tenon frame construction. Initially, background information is needed
concerning the difficulties encountered in cutting round tenons and
mortises of a size suitable for use with small-diameter roundwood, the
ease of assembly with round mortise and tenon frame construction, and
the integrity of the assembled frame. To obtain insights into whether
this construction is practicable, as well as further insights into the
specific characteristics and peculiarities of round mortis e and tenon
joint building frame construction, a small building was designed and
constructed utilizing this construction system. A description of the
building and the lessons learned from its construction are given in this
paper.

MATERIAL

All of the material used in construction of the frame was obtained
from a study conducted by Serrano (1999) on the effect of longitudinal
growth strains on lumber warp in material cut from small-diameter
yellow-poplar logs. The larger structural members obtained from this
study measured a nominal 3-1/2 inches square. A part of these members
had been cut as is from small-diameter yellow-poplar stems and included
the core of the tree in the center of the cross section. The remainder
of the members were fabricated by Serrano (1999) by sawing squared stems
lengthwise into two identical parts and then gluing the outside broad
faces back to back, which places the core of the stem on two opposite
outside faces of the resulting square timber. Smaller members, measuring
1-1/2 by 3-1/2 inches in cross section, were cut from small-diameter
stems without regard to core position; i.e., no attempt was made to
position core material in a specific position in the end of the member.

MACHINING OF MEMBERS

Tenons for the frame were cut with 2-inch diameter by 12-inch long
deep hole saws produced by a commercial supplier. A simple low cost
"drill press" was developed to machine the tenons (Fig. 1). In
practice, the squared tree stem, or other structural member, was first
positioned and secured in place in the machine. The hole saw was then
advanced into the end of the member until the desired depth of cut
(tenon length) was obtained. The excess material was then removed from
around the tenon with a handsaw in order to leave a uniform rectangular
shoulder on the member. Diameter of the tenons at the time of machining
was a nominal 2 inches.

Mortises were drilled in the members with 2-1/16-inch-diameter
Forstner bits. Diameter of the holes measured a nominal 2-1/16 inches.

FRAME DESIGN AND CONSTRUCTION

Drawings of the frame developed during the study are given in
Figures 2 and 3. Overall, the frame measured 6 by 12 feet in cross
section by 9 feet tall. These dimensions were chosen in order to allow
the use of material that had previously been processed from
small-diameter timber.

All of the material was first cut to size. Tenons were then cut on
the ends of the members and mortises drilled into the sides of the
members as needed.

Construction of the frame itself began with the insertion of the
floor joist tenons into the corresponding mortises in the sills. Once
this was done, the resulting floor system consisting of floor joists and
sills was secured on the foundation. The corner post tenons were then
inserted into the corner sill mortises. Since the side sills were
constructed of two pieces, wall stud tenons were inserted into the two
overlapping sill joints to join these two members together. The floor
joist tenons on either side of this joint were then pinned to the sills.
The remaining wall studs and doorposts were then added by inserting
their tenons into corresponding mortises in the sills.

The tenons of the window headers and sills were then inserted into
their corresponding mortises cut in the wall studs. Next, the front and
back top plates were slipped in place over the tops of the front corner
and door tenons and the back wall corner and stud tenons and seated on
the shoulders of the tenons. The tie beam was installed in a similar
manner. The side rafter plates were then slipped in place over the ends
of the sidewall stud tenons and the corner post tenons (Fig. 4).

The ridge beam support columns, or king posts, were next inserted
in place. Then, the rafters were slipped into place over the tops of the
wall stud tenons while the rafter tenons simultaneously were slipped
into corresponding mortises in the ridge beam. Each rafter was
"pinned" to its corresponding wall stud tenon with a 12-penny
nail. It should be noted that the outer faces of the ridge beam were cut
at an angle so that they were perpendicular to the longitudinal axes of
the rafters. The ridge rafter beam along with the side rafter plates
were extended beyond the front and back plates in order to support fly
rafters needed to provide an overhang for the front and back of the
structure (Fig. 5). Finally, the facia headers were added.

After the frame was erected, the walls and roof were sheathed with
appropriate standard grades of plywood and the roof was covered with
asphalt shingles. Treated facia boards were used to cover the exposed
face of the facia headers. Short lengths of facia were also installed in
each gable end. Finally, the building was floored with additional
material salvaged from small-diameter yellow-poplar stems.

Construction of the frame is such that the sills are locked
together at the corners by the corner post tenons. These joints locate
the corners of the structure and provide resistance to lateral loading
of the sills. Likewise, the sills are linked together at the mid-length
points by the tenons of the wall studs supporting the tie beam. If the
sills are not fastened to a secure foundation, the floor joist tenons
that frame into the sills on either side of the intermediate sill joint
are pinned to the sills in order to provide resistance to lateral
loading.

The front and back top plates along with the tie beam provide
resistance to lateral forces applied to the walls by the side rafter
plates and also locate the position of the tops of the corner posts
spatially in a side-to-side direction.

Vertical forces applied to the ridge rafter beam by the rafters are
transferred to the front and back top plates and the tie beam by the
ridge beam support posts, or, king posts. Hence, the ridge beam roof
loads are transferred to the top plates at their center points. For
longer spans, intermediate beam and column construction can be used so
that these loads would be transmitted to the third or quarter points of
the end plates and tie beams as desired. The rafters are notched and
mortised so that they slip over the tenons of the wall studs and seat on
the top surface of the side rafter plates. The side rafter plates,
therefore, carry a large part of the vertical roof load and also provide
resistance to the horizontal components of the roof load.

DISCUSSION

The frame was first erected within the Wood Research Laboratory,
disassembled, transferred to the construction site, and then re-erected.
Time to disassemble was about 15 minutes; time to re-assemble was about
40 minutes. The consensus of those participating was that structures
such as this could be assembled easily and rapidly from a stock of
standardized parts, essentially without the use of any tools.

It was found that when parts with matching tenons and mortises were
allowed to dry, the mortises often shrank more than the tenons. The
closeness of fit between tenons and mortises is quite important since
the tighter the joint the stiffer the structure. On the other hand, the
tighter the fit, the more difficult the assembly of the frame. Unequal
shrinkage could be a serious problem with parts precut from green or
partially seasoned wood and stored for significant periods of time
before assembly. Ideally, such parts should be cut from seasoned wood.

Experience with the simple horizontal drill press indicates that
round tenons can be cut both easily and quickly with the simple
equipment developed. In general, the tenons are round and true. Mortises
are cut easily with Forstner type bits. Exploratory tests indicate that
the same equipment can be used equally well with 3- and 4-inch-diameter
hole saws.

Additional work is underway to develop other designs and
constructions. Roof frame designs and roof frame systems are of
particular importance. Work is also underway to obtain information
concerning the bending strength, lateral shear strength, and pinned
strength of round mortise and tenon joints since this information is
critical for the rational design of frame systems based on round mortise
and tenon joint construction.

CONCLUSIONS

Results of this exploratory building project indicate that round
mortise and tenon joinery provides a simple straight-forward method of
constructing building frames. Erection techniques are simple and only
basic tools are needed. Furthermore, basically unskilled labor can be
used to erect a frame.

The system lends itself to the production of standard parts that
can be manufactured locally by cottage level industry or regionally by
major producers. Standardized parts can be easily incorporated into
modular constructions to serve a wide variety of needs.

On-site frame assembly times are short and assembly can be
accomplished with unskilled labor. The system is thus well suited to the
rapid solution of widespread building construction needs.

Importantly, the system is well suited for the use of large amounts
of small-diameter tree stems for which there is now little use.
Furthermore, costly connectors are eliminated because they are integral
parts of the members themselves.

In summary, round mortise and tenon construction joinery provides a
means of utilizing low-value wood material in the construction of useful
building frames. Some of these building frames might be used to satisfy
the simple need for backyard storage sheds in developed countries, while
others might be used to provide the basic framework for shelter for the
less fortunate, for farm buildings, and light industrial buildings.